1. Field of the Invention
The present invention relates to a dielectric lens, a dielectric lens antenna including the same, and a wireless device including the same. More specifically, the present invention relates to a dielectric lens applied for a motor-vehicle-mounted radar which uses millimeter-waves, a dielectric lens antenna including the same, and a wireless device including the same.
2. Description of the Related Art
With the recent advance of motor-vehicle-mounted radar, control of the directivity of an antenna has been a significant concern.
FIGS. 11A, 11B and 11C show a prior art dielectric lens. FIG. 11A is a plan view, FIG. 11B is a front view, and FIG. 11C is a side view. In a dielectric lens 1, a lens 2 is substantially in a shape formed by cutting a part of a sphere. In the plan view, it is formed rotation-symmetrically, namely, in a round form, and in the front view and the side view, it is formed in a circular form. The lens 2 is made of dielectric materials such as ceramics, resin, plastic, or their composite materials. The focal direction of the dielectric lens 1 is the -z-axis direction.
FIGS. 12A, 12B and 12C show a dielectric lens antenna including the dielectric lens 1 shown in FIG. 11A, 11B and 11C. FIG. 12A is a plan view; FIG. 12B is a front view; and FIG. 12C is a side view. In FIG. 12, the dielectric lens antenna 5 is formed by disposing a primary radiator 7 at the focal point 6 of the dielectric lens 1.
FIG. 13 shows a conceptual view (a front view) illustrating the directivity of a beam radiated from the dielectric lens 1 of the dielectric lens antenna 5 shown in FIGS. 12A, 12B and 12C. In FIG. 13, the same reference numerals are given to the same parts as those in FIG. 12 or the equivalent parts to those in FIG. 12; their descriptions are omitted. As shown in FIG. 13, the shape of beam 3 radiated from the dielectric lens 1 of the dielectric lens antenna 5 is a pencil-beam shape on the x-z side. In this case, the length (the height in FIG. 13) of z-axis direction of the beam 3 indicates the magnitude of a gain of the dielectric lens antenna 5, and the width of the beam 3 indicates the magnitude of the beam width of the dielectric lens antenna 5.
As seen above, the gain of the dielectric lens antenna 5 amounts to a maximum value in the z-axis direction. With respect to the z-axis direction, the angle in which a gain decreases by 3 dB from the maximum value, namely, the angle in which the gain amounts to a half is referred to a half-value angle, which indicates the directivity of the antenna. The shape of the beam 3 radiated from the dielectric lens 1 of the dielectric lens antenna 5 is the same on all the sides which include the z-axis and parallel to the z-axis, such as the x-y side, so that the line connecting points of the half-value angles forms a round form when viewed from the front of the dielectric lens antenna 5. In addition, the half-value angle is substantially indicated by a formula: EQU A half-value angle (.theta.)=70.lambda./D
(.lambda.: wavelength of the used frequency, PA1 D: antenna-aperture diameter)
Thus, a half-value angle is inversely proportional to an antenna aperture diameter. In contrast, the wider the aperture diameter, the larger the gain.
A motor-vehicle-mounted radar does not necessarily require the information of a vertical direction (up-and-down directions) with respect to a traveling direction of a motor vehicle. On the contrary, in order to prevent malfunctions due to reactions with a pedestrian overpass or a viaduct, it may be better for the radar to have less information of a vertical direction. Meanwhile, the information of a horizontal direction (a traveling direction and right-and-left directions of a motor vehicle) is primarily necessary, since other motor vehicles and obstacles are targeted. This can require a wide-angle antenna, in which a beam is narrowed in the vertical direction, whereas it is widened in the horizontal direction. In this case, in order to widen the beam, namely, to widen the half-value angle, it is necessary to make the antenna-aperture diameter smaller, namely, it is necessary to reduce the diameter of the dielectric lens. However, reducing the diameter of the dielectric lens leads to decrease in gain, thereby it creates a problem in which the lens can only detect at close range when it is used in radar. In addition, reducing the diameter of the dielectric lens leads to extension of the beam not only in the horizontal direction but also in the vertical direction; it thereby leads to further decrease in the gain in the horizontal direction.